A drill frequently used as a hole machining cutting tool is disposed with a cutting edge at an axial tip and a chip discharge flute in an outer circumferential surface in an axial direction and is rotated around an axial center to perform cutting with the cutting edge at the tip while discharging chips through the chip discharge flute.
Drills described in Patent Documents 1 and 2 are examples thereof. These drills have an inner circumferential portion of a cutting edge formed into a concave curve and an outer circumferential portion of the cutting edge formed into a convex corner shape to define an obtuse intersection angle between an inner wall surface of the chip discharge flute and a margin portion on a drill cross section and is considered to achieve curling of chips and an improvement in strength of the outer circumferential portion of the cutting edge and to have drill durability. However, since a corner convex portion is disposed on the outer circumferential portion of the cutting edge, generated chips tend to be segmented at a portion corresponding to the corner convex portion, tangling with each other to generate clogging of chips, and since the chips cut on an outer circumferential side relative to the corner convex portion are caused to flow toward an outer circumference, poor curling property and low discharge property may make a resistance applied to the drills larger, compromising the drill durability.
In this regard, a drill described in Patent Document 3 has been proposed. This drill has a convexly-curved cutting edge portion formed into a convexly-curved shape convexed in a drill rotation direction on an outer circumferential end side of a cutting edge and has a concavely-curved cutting edge portion formed into a concavely-curved shape concaved in the drill rotation direction on an inner circumferential side of the convexly-curved cutting edge portion, and the convexly-curved cutting edge portion and the concavely-curved cutting edge portion are smoothly continued. Therefore, an obtuse intersection angle is made between the cutting edge and a margin portion on an outer circumference of a drill main body, increasing a strength to prevent cracking and chipping from occurring, and since the chips cut by the cutting edge are not segmented at the inner/outer circumferences of the cutting edge and the chips are rolled into the inner circumferential side and sufficiently curled by the convexly-curved cutting edge portion, the chips are smoothly discharged and the tool durability is considered to be enhanced.